A major objective of the proposed research is to provide new chemical information about iron-sulfur complexes in which the ligand donor atoms belong to functional groups commonly found in the iron- sulfur proteins. Synthetic efforts will be directed towards the preparation of iron-mercaptide or sulfide oligomers, since chromophores of this type are known to comprise the important functional units at the active sites of these proteins. Multidentate chelating sulfur ligands, including small cysteine-containing polypeptides, will be employed in an attempt to prepare stable complexes incorporating the FenSn moiety, n greater than or equal to 2. By studying the relationship between the molecular geometry (including degree of polymerization) and electrochemical behavior (as judged by electronic and esr spectroscopy and by cyclic voltammetry) for these iron-sulfur complexes, our model for the low and varying redox potentials in the proteins will be tested. Similar studies involving chemical modification through reconstitution of the apo-ferredoxin proteins will be carried out. The reduction potential and specific activity of spinach ferredoxin are being studied as a function of medium conditions.